In a conventional fan, in particular, in a centrifugal fan, two-dimensional blades in each of which no twisted form exists between a main plate and a shroud have been of a mainstream technique. Further, in pursuing reduction of noise and power consumption, a fan including three-dimensional blades with twisted forms among main plates and shrouds appeared and advance of performance has been sought.
Since the fan having the three-dimensional blades is able to address problems of noise, power consumption or others, it became possible to increase rotation speed. However, in general, a problem that a rotation sound (NZ sound) is likely to be generated has occurred by increasing the rotation speed of the fan.
Therefore, conventionally, for example, a technique for reducing the rotation sound by adopting a structure in which a minimum common divisor of combinations of the number of blades and a pitch angle of blade becomes a maximum value has been suggested (see, for example, Patent Literature 1).
Moreover, a technique for reducing the rotation sound by forming a leading edge shape of blade into a shape different by blade and arranging the blades so that a resultant vector of weight moment forces of respective blades becomes minimum is suggested (see, for example, Patent Literature 2).